Widely known in the past has been a control system of an internal combustion engine which is provided with an air-fuel ratio sensor in an exhaust passage of an internal combustion engine, and controls the amount of fuel supplied to the internal combustion engine based on the output of the air-fuel ratio sensor. As such a control system, one which is provided with an air-fuel ratio sensor at the upstream side of the exhaust purification catalyst provided in the engine exhaust passage and is provided with an oxygen sensor at the downstream side thereof is known (for example, PTLs 1 to 4, etc.)
For example, in the system described in PTL 1, feedback control is performed, based on the output of the upstream side air-fuel ratio sensor, so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the stoichiometric air-fuel ratio. In addition, since deviation can occur in the output of the upstream side air-fuel ratio sensor, the output of the upstream side air-fuel ratio sensor is corrected based on the output of the downstream side oxygen sensor. Further, a correction amount of the output of the upstream side air-fuel ratio sensor based on the output of the downstream side oxygen sensor is incorporated to a learning value by a certain ratio every certain time interval, to update the learning value. The learning value is used for correction of the output of the upstream side air-fuel ratio sensor.
In addition, in the system described in PTL 1, when the mechanical compression ratio which is set by the variable compression ratio mechanism is high, the time interval for incorporating to the learning value made shorter and the ratio of incorporating to the learning value is made larger so as to increase the speed of incorporating to the learning value. Due to this, according to the system described in PTL 1, even when the mechanical compression ratio is high and thus the ratio of the unburned HC contained in the exhaust gas is high, it is made possible to make the learning value quickly converge.